Placing a condition while calculating using numpy array

Question

Below is a short piece of code that for some reason keeps generating the following value error message: ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all().

import numpy as np
p=np.array([1,2,3])
q=np.array([4,5,5])

while p + q==7:
        try:
            assert p.any()
            assert q.any()
        except AssertionError:
            print('(p + q)<=6')
        print  (p + q)

I have tried both p.any and p.all, still getting the same error message. Any suggestions? Thanks.

chthonicdaemon · Accepted Answer · 2013-08-23 08:16:12Z

4

Your problem is that p and q have three elements each, so p + q == 7 will also have three elements. For the while loop, you need something that can be interpreted as True or False - the error is telling you that three elements can't be interpreted as True or False without more information: it's ambiguous. If you want all of the elements to be equal to 7, use

while np.all(p + q == 7):

if you want any of them to be equal, use

while np.any(p + q == 7):

answered Aug 23, 2013 at 8:16

chthonicdaemon

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5 Comments

Sven Marnach Over a year ago

Alternatively, do exactly what the error message suggests: while (p + q == 7).any(): etc.

chthonicdaemon Over a year ago

For me, the function scans closer to English than the method in this case ("while all elements equal to 7").

Sven Marnach Over a year ago

I was mainly trying to give a hint what the error message means.

Tiger1 Over a year ago

Hi all, thanks for your suggestions. I actually made used of (p + q==7).any(), but I placed it inside the try and except statement.

J. Martinot-Lagarde Over a year ago

Another remark: the error gave you a line number (line 5 if your code is exactly the same) but you looked 2 lines down. Read carefully the error messages next time, they give precise indications about the problem.

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